A need exists for an improved vegetable oil with a significantly extended shelf life and greater heat stability relative to generic canola oil and a positive nutritional contribution to animal, including human, diets.
Canola oil has the lowest level of saturated fatty acids of all vegetable oils. "Canola" refers to rapeseed (Brassica) which has an erucic acid (C.sub.22:1) content of at most 2 percent by weight based on the total fatty acid content of a seed, preferably at most 0.5 percent by weight and most preferably essentially 0 percent by weight and which produces, after crushing, an air-dried meal containing less than 30 micromoles (.mu.mol) per gram of defatted (oil-free) meal. These types of rapeseed are distinguished by their edibility in comparison to more traditional varieties of the species.
As consumers become more aware of the health impact of lipid nutrition, consumption of canola oil in the U.S. has increased. However, generic canola oil cannot be used in deep frying operations, an important segment of the food processing industry.
Canola oil extracted from natural and previously commercially useful varieties of rapeseed contains a relatively high (8-10%) .alpha.-linolenic acid content (C.sub.18:3) (ALA). This trienoic fatty acid is unstable and easily oxidized during cooking, which in turn creates off-flavors of the oil (Gailliard, 1980, Vol. 4, pp. 85-116 In: Stumpf, P. K., ed., The Biochemistry of Plants, Academic Press, New York). It also develops off odors and rancid flavors during storage (Hawrysh, 1990, Stability of canola oil, Chapter 7, pp. 99-122 In: F. Shahidi, ed. Canola and Rapeseed: Production, Chemistry, Nutrition, and Processing Technology, Van Nostrand Reinhold, New York). One such unsatisfactory species heretofore has been Brassica napus, i.e., spring canola, a type of rapeseed.
It is known that reducing the .alpha.-linolenic content level by hydrogenation increases the oxidative stability of the oil. Hydrogenation is routinely used to reduce the polyunsaturates content of vegetable oils, thereby increasing its oxidative stability. The food industry has used hydrogenation to raise the melting point of vegetable oils, producing oil-based products with textures similar to butter, lard and tallow. Trans isomers of unsaturated fatty acids are commonly produced during hydrogenation. However, the nutritional properties of trans fatty acids mimic saturated fatty acids, thereby reducing the overall desirability of hydrogenated oils (Mensink et al., New England J. Medicine N323:439-445, 1990; Scarth, et al., Can. J. Pl. Sci., 68:509-511, 1988). Canola oil produced from seeds having a reduced .alpha.-linolenic acid content would be expected to have improved functionality for cooking purposes with improved nutritional value, and therefore have improved value as an industrial frying oil.
However, in general, very little variation exists for .alpha.-linolenic acid content in previously known canola quality B. napus germplasm (Mahler et al., 1988, Fatty acid composition of Idao Misc. Ser. No. 125). Lines with levels of .alpha.-linolenic acid lower than that of generic canola oil are known, but have sensory, genetic stability, agronomic or other nutritional deficiencies. For example, Rakow et al. (J. Am. Oil Chem. Soc., 50:400-403, 1973), and Rakow (Z. Pflanzenzuchtg, 69:62-82, 1973), disclose two .alpha.-linolenic acid mutants, M57 and M364, produced by treating rapeseed with X-ray or ethylmethane sulfonate. M57 had reduced .alpha.-linolenic acid while M364 had increased .alpha.-linolenic acid. However, the instability of the fatty acid traits between generations was unacceptable for commercial purposes.
Brunklaus-Jung et al. (Pl. Breed., 98:9-16, 1987), backcrossed M57 and other rapeseed mutants obtained by mutagenic treatment to commercial varieties. BC.sub.0 and BC.sub.1 of M57 contained 29.4-33.3% of linoleic acid (C.sub.18:2) and 4.9-10.8% of .alpha.-linolenic acid (C.sub.18:3). The oleic acid (C.sub.18:1) content was not reported, but by extrapolation could not have exceeded 60%.
Four other lower .alpha.-linolenic acid canola lines have been described. Stellar, reported by Scarth et al. (Can. J. Plant Sci., 68:509-511, 1988), is a Canadian cultivar with lower .alpha.-linolenic acid (also 3%) derived from M57. Its .alpha.-linolenic acid trait was generated by seed mutagenesis. S85-1426, a Stellar derivative with improved agronomic characteristics, also has lower (1.4%) .alpha.-linolenic acid (Report of 1990 Canola/Rapeseed Strain Test A, Western Canada Canola Rapeseed Recommending Committee). IXLIN, another lower .alpha.-linolenic acid (1.8%) line described by Roy et al. (Plant Breed., 98:89-96, 1987), originated from an interspecific selection. EP-A 323 753 (Allelix) discloses rape plants, seeds, and oil with reduced .alpha.-linolenic acid content linked to limitations in the content of oleic acid, erucic acid, and glucosinolate.
Another nutritional aspect of rapeseed, from which canola was derived, is its high (30-55 .mu.mol/g) level of glucosinolates, a sulfur-based compound. When the foliage or seed is crushed, isothiocyanate esters are produced by the action of myrosinase on glucosinolates. These products inhibit synthesis of thyroxine by the thyroid and have other anti-metabolic effects (Paul et al., Theor. Appl. Genet. 72:706-709, 1986). Brassica varieties with reduced glucosinolates content (&lt;30 .mu.mol/g defatted meal) were developed to increase the nutritional value of canola meal (Stefansson et al., Can. J. Plant Sci. 55:343-344, 1975). Meal from an ultra-low glucosinolates line, BC86-18, has 2 .mu.mol/g total glucosinolates and significantly improved nutritional quality compared to generic canola meal (Classen, Oral presentation, GCIRC Eighth International Rapeseed Congress, Saskatoon, Saskatchewan, Jul. 9-11, 1991). Neither its fatty acid composition nor its seed glucosinolates profile is known.
There remains a need for an improved canola seed and oil with very low .alpha.-linolenic levels in the oil and low glucosinolates in the seed to significantly reduce the need for hydrogenation. The .alpha.-linolenic content of such a desirable oil would impart increased oxidative stability, thereby reducing the requirement for hydrogenation and the production of trans fatty acids. The reduction of seed glucosinolates would significantly reduce residual sulfur content in the oil. Sulfur poisons the nickel catalyst commonly used for hydrogenation (Koseoglu et al., Chapter 8, pp. 123-148, In: F. Shahidi, ed. Canola and Rapeseed: Production, Chemistry, Nutrition, and Processing Technology, Van Nostrand Reinhold, New York, 1990). Additionally, oil from a canola variety with low seed glucosinolates would be less expensive to hydrogenate.